Without permission, you are still free to use facts and other non-creative information. Of course, you still must provide credit to the source of information in your own guide; if you don't, you are plagiarizing.

For example, you could get the hit points for the enemies in a game from the official strategy guide for that game and post them in your own guide in your own format. These are facts, and not protected by copyright. However, if you were to copy the guide's strategy on how to beat the boss, that would be a copyright infringement, as you are copying their creative work. Copying the format of the table of hit points would also be a violation of their copyright.

What is Plagiarism?

Simply put, plagiarism is the taking of information from a source and claiming it as your own. If you read a boss strategy in another guide and re-create in your own words, if you don't credit the original source of the information, you're plagiarizing.

Copyright infringement is not always plagiarism, and plagiarism is not Copyright infringement. One is a legal matter, the other is an ethical matter. Both should be avoided at all costs when creating your guide.